Cell Types - The Three Major Cell Types
Prokaryotic Cells are the simplest cells and scientists believe they were the first cells to inhabit our world. They are much smaller than eukaryote cells, as they lack a defined nucleus and most other organelles that are found in eukaryotes. In fact, the word "prokaryote" means "before the nucleus." There are two types of prokaryotes - bacteria and archaea.
The nuclear substance of prokaryotes consists on a single chromosome which is in direct contact with the cytoplasm of the cell. There is no defined membrane surrounding the nuclear region, which is called a "nucleoid" in these cells.
Three primary features found in prokaryotic cells are:
1. Flagella and Pili - protein structures which project from the cell's surface which are used primarily for movement. They are also used to help cells attach to one another. Flagella and pili are not found in all prokaryotes.
2. Envelope - a cell wall which covers a plasma membrane. The envelope of the cells is the containing structure of the cell, separating the interior of the cells from the outer environment. Some bacteria also have an additional outside later of cells, called a capsule.
3. Cytoplasmic Region - the area contained inside the cell envelope or capsule. The cell DNA and ribosomes are found in this area. The DNA of bacteria is generally circular in shape. Some prokarotes carry additional extrachromosomal DNA inclusions called "plasmids." Plasmids are also circular in shape, and the generally carry out additional functions within the cell, such as antibiotic resistance.
Eukaryotic cells, found in plants and animals, are more advanced than prokaryotic cells, and are more advanced in structure. They are about 15 times wider than the average prokaryote, and can have a cell volume as much as 1000 times greater.
The primary difference between peukaryotes and prokaryotes is that the eukaryotic cell has a defined cell nucleus, which contains the cell's DNA. The word "eukaryote" means "true nucleus."
Additional primary features of the eukaryotic cell are:
1. Cell Membrane - the plasma membrane that makes up the outer boundary of the cell. In addition to the cell membrane, plant cells also have a cell wall.
2. Chromosomes - the DNA in eukaryotes is organized into linear molecules called chromosomes. These chromosomes are stored in the nucleus in the cell.
3. Primary Cilia - protein structures on the outside of the cell which serve as sensory organs. Eukaryotes use these cilia to sense temperatures, movement and chemical makeup of their environment.
4. Flagella or Motile Cilia - more complete than those found in prokaryotes, but which perform similar functions in controlling the mobility of the cell.
5. Organelles - eukaryotic cells may also contain various "small organs" which perform specific cell functions. Various organelles include: nucleolus, ribosomes, vesicles, rough endoplasmic reticulum, Golgi body (or "apparatus"), cytoskeleton, smooth endoplasmic reticulum, mitochondrion, microtubules, vacuole, cytosol, lysosome, and centriole.
Mesokaryotic cells share characteristics of both prokaryotic and eukaryotic cells. The first part of the word, 'meso,' means 'in between', while 'karyon' means 'nucleus.'
Mesokaryotic cells exhibit a well-organized nucleus, as eukaryotes do, but it's nucleus divides through a process called amitosis, which more closely resembles the behavior of prokaryotic cells. The nucleus of a mesokaryotic cell duplicates itself with one nucleus going with each cell half when the rest of the cell divides.
A group of organisms known as "dinoflagellates", marine plankton and algae, are generally considered to be examples of mesokaryotes.
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